[AR] Re: hydrogen and oxygen propellant produced on Mars
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Tue, 20 Jun 2017 16:11:00 -0400 (EDT)
On Tue, 20 Jun 2017, John Dom wrote:
As for returning to Earth will the H2 & O2 propellant produced in situ
require liquefaction (cooling) in its tanks on Mars? Or could the gaseous
state Isp suffice?
Gaseous-state Isp is potentially actually slightly better, because there's
a bit more thermal energy in the propellants at the start. The problem is
the terribly low density, which makes for poor engine T/W and impossibly
heavy tanks. A one-cubic-meter tank holds about 70kg of LH2, but even at
1500psi, only about 8kg of GH2 at 25degC. The same tank holds 130kg of
GOX at 1500psi and 25degC, or 1140kg of LOX.
Gaseous propellants are not ridiculous for small RCS systems and for
spacecraft that need small amounts of maneuvering. For serious delta-V,
the propellants *have* to be liquids -- there's no way to get reasonable
mass ratios at gas densities.
Henry
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